Electric-current transmission



"May 25 1926.

J. L. FEARING ELECTRIC CURRENT TRANSMISS ION Filed Dec. 8. 1923 4 Sheets-Sheet 1 Wye/72W. Jaw/7 LEW/77y,

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- May 25,1926. 1,585,842

J. L. FEARING 2 ELECTRIC CURRENT TRANSMIS SION Filed Dec. 8. 1923 4 SheetsSheet 2 May 25, 1926. 1,585,842

J. L. FEARING ELECTRIC CURRENT TRANSMIS SION Filed Dec. 8, 1923 4 Sheets-Sheet 3 Wye/72W J05/7/7 A. Ear/0y,

tion and to block transmission in the oppo- These and various other fea-,

Patented May 25, .1926.

Unit STATES JUSTIN IL. FEARING, 'OF SOUTH WEYMOUTH, MASSACHUSETTS, ASSIGNOR TO WESTERN- ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

I ELECTRIC-CURRENT TRANSMISSION.

This invention relates to the transmission amplifying paths so connected to the input of the other of these two paths as to deliver to the grid of an electron discharge device in this other path a potential tending to prevent the transmission through this other. path, to thereby prevent singing of the re-' peating elements due to currents from one of the amplifying 'pathscirculating through the oppositely directed amplifying path, without requiring either networks for balancing the impedence of the lines or mechanical switching means at the repeater. The repeater may be connected in a line to which is connected means for sending either alternating signal current alone or direct current signals alone over the line to the repeater, for the transmission of signals through the repeater; or this sending means may transmit alternating signal currents to the line and simultaneously send to the linesteady direct current to provide a control potential for conditioning the repeater to faithfully repeat the alternating currents in the desired direction and to block transmission in the opposite direction. In' one form of the invention shown in the drawings, alternating current transmitted over the line to the repeater may be amplified and rectified at the repeater and applied to the repeaters as a control potential for conditioning the repeater to faithfully repeat the alternating-current in the desired direcsite direction. tures of the invention will be by reference to the drawings.

Fig. 1 shows schematically a signaling system embodying one form of the invention; Fig. 2 is a curve for facilitating description of the operation of repeaters used in accordance with the invention; Fig. 3 is a circuit diagram of one form of twoway repeater embraced in the invention; Figs. 4, 5 and 6 are circuitdiagrams of pointed out .other forms of two-way repeaters embraced in the invention; Fig. 7 is a diagrammatic showing. ofa type of repeaterwhich' may employ various combinations of the oneway repeaters shown in Figs. 1, 3, 4:, 5 and 6; Fig. 8 is a table indicating certain repeater combinations which may be employed in Fig, 7 and in Fig. 11; Fig. 9 is a circuit diagram of a signaling system embodying a form of the invention; Fig. 10 is a diagrammatic showing of a general type of repeater a form of which is shown in Figg'9'and which may emplo a plurality of any one of various forms 0 two-way repeaters, such as those shown in Figs. '1, 3,

4 and 5;-Fig. 11 is a diagrammatic showing of a general type of repeater which may be composed of various forms of repeaters, such as the forms shown in Figs. 1, 3, 4, 5, and 6; Fig. 12 is a circuit diagram of still another form of repeater embraced in the invention; Fig. 13 is a schematic showing of a signaling system embodying a form of the invention; and Fig. 14-is a similar showing of a signaling system embodying still another form of the invention.

. Referring to Fig. 1, a line L transmits signals between a signaling station at its west end comprising apparatus A, B, C and D, and a similar station at its east end comprising apparatus A, B, C, D, which are the same as apparatus A, B, C, and D, respectively.

A repeater circuit M, comprising two oppositely pointed one-way repeaters such, for example, as those designated S, is connected in the line for amplifying currents transmitted in either direction over the line. The- 'normally'the space current in the tubes is reduced to substantially zero value. other words, in the absence of any trans mission over line L the values of the gridpotentials and the space currents of the re eaters are in the neighborhood of those indicated by the adjustment designated a in Fig. 2', w 'ch shows a plate-current gridpotential graph typical of such curves for repeaters of the character of repeaters S.

With the repeaters so adjusted, they will not sing, even when direct current or alternating current of any character is transmitted to the repeater circuit from either the west or the east; for in each repeater the value of the plate current is not subject to change by changes of grid potential as long as the grid potential is sufliciently negative to pre vent flow of space current, and whenever space current flows in either repeater it makes the grid of the other repeater more negative. The space current path for the west repeater extends from the positive pole of the plate current battery shown in that repeater, through the space discharge path in the repeater to the filament, west and over. the lower side of line L to theapparatus-at the west end of the line, through this apparatus as explained hereinafter, and eastward over the upper side of the line L to the negative pole of the plate current battery, Similarly, the space current path for the east repeater is through the apparatus at the east end of line L.

Resistances 1 are connected across the out-- put leads of each repeater, and their value is preferably so high that very little of the space current of the repeaters passes through them. It will be noted that these resistances also act as grid leak resistances across the input leads of the repeaters; Condensers 2 of small capacity are preferably connected across line L at each end of the repeater circuit M. The function of resistances 1 and condensers 2 is to prevent singing of the repeater due to slight disturbances in the line or to the feeding back from the line to the repeater of energy repeated into and stored in any reactance in the line or load. The value of resistances 1 should be sufficiently high to prevent undue loss because of their shunting efiects upon the outputs of the repeaters. Capacities 2 are adjusted to be large enough to prevent singing and at the same time are kept as small as possible to avoid shunting the line signals. At

high frequencies, it may be desirable to switch capacities 2 out of circuit, and to utilize merely the inherent capacity of the circuit. I

The apparatus A comprises a key 3 which can be closed for transmitting positive potential from a battery 4 through a variable resistance 5 and the line L to the grid of the east repeater. By shifting the switch 6 to the position opposite to that shown in the drawing, the potential applied by battery 4 to the grid of the east repeater can be reversed. However, it should be noted that when the repeater M is of the form shown 111 Fig. 1 negative impulses will not be repeated. The apparatus A may also comprise a key 7 which can be closed for transmitting I from a battery 8 through a variable resistance 9 to the grid of the east repeater either negative or positive impulses, according to whether the switch 6 is set in the position shown or in the opposite position. The line L, where it connects with apparatus A, is bridged by a resistance 10. The value of this resistance, together with that of the line i and a polarized double-throw relay 13 normally restoring to a neutral position and selectively operable in one direction or the other in accordance with the sign of the incoming signal, to energize one or the other of the relays 14 and 15, which may be telegraph sounders, for example. The relay 13 is so connected to a resistance 16 in the space current circuit of amplifier 12 and to a resistance 17 in circuit with the space current battery of tube 12 that normally no current flows through relay 13, but upon change of potential of the grid of the tube, due to closure of switch 3 or 7, or the corresponding switches in A, the potentials applied to the winding of relay 13 from resistan'ces 16 and 17 are unbalanced and the relay is operated to its upper or to its lower contact. The grid potential of tube 12 is normally adjusted to the value indicated by line 0 in Fig. 2, so that if key 3, for example, be closed, the current in 16 will increase and current will flow in one direction through the winding of relay 13 and cause the energization of, say, relay 14. Upon the release of key 3, relay 14: will be deenergized. If key 7 be closed the space current of tube 12 will decrease, current will flow through the winding of relay 13 in a direction opposite to that just de scribed and relay 15 will be energized. lVhen repeating from A to B, either relay 14 or 15 may receive, since the switch 6 may, at will, change the polarity of the potentials delivered to the grid of tube 12 by repeater M.

The apparatus C is a multiplex sending apparatus for transmitting to the receiving station D at the east end of the line L. As shown, this apparatus comprises a channel 20 2Q for either sending telephone messages from transmitter 21 when switch 22 is on its right hand contact, or sending telegraph messages from buzzer 23 when switch 22 is on its left hand contact. The sending apparatus C, as shown, also comprises a carrier current channel 25 for sending either telephone or telegraph signals, this channel including an alternating current generator '26 anda modulating tube 27, for generating alternating current waves and modulating them by the signal waves in a well known manner. The apparatus C, as shown, also comprises a carrier current channel 28 for sending either telephone or telegraph signals, the channel 28 being similar to the channel 25 except, of course, that the carrier frequency employed in the two: chan nels is difi'erent. Each of the sending channels includes a suitable band filter BF for passing frequencies to be transmitted from that channel and excluding other frequencies, and the channels are all connected in parallel with each other and through a transformer 29, an amplifier 30 and a reversing switch 31 to the line L. A resistance 32 is bridged across the output of amplifier 30. The value of this resistance should be such that it will form a suitable path for the direct current space current of tube 30.

The apparatus D is a multiplex receiving apparatus for receiving messages transmitted from the apparatus (Y, and coinprises an amplifier 33 with its grid and filament connected across the line L, the

I output of the amplifier being connected through a transformer to three receiving channels 34, 35 and 36. Each of the channels includes the usual band filter BF for passingfrequencies to be received by that channel and excluding other frequencies, and also includes receiving means R of any suitable type for receiving the telegraph or telephone messages sent from station C, the channels 35 and 36 being the carrier current channels and therefore including detecting means D between the filters BF and the receiving means R The steady grid potential for the grid of tube 33 is supplied from the grid battery for that tubethrough a high resistance 37.

One way in which the system may be operated is to transmit positive impulses of current to the grid of the east repeater by operating either the'key 3 or the key 7, ac cording as the switch 6 is positioned to the right, as shown, or to the left, respectively. These impulses are repeated through the east repeater with reversal of sign, thereby applying negative impulses from the re peater to the sounder apparatus at the 7 east end of the line in such manner as to operate one sounder or the other in ac cordance with the position of switch 6'. At

the same time, one or the other of the sounders 14 or 15 at the west end of the line will be operated, depending upon whether the transmitting key used is key 3 or key 7, as described above. Thus a telegraph message may be transmitted, the switches 31 and 31 preferably, though not neces sarily, being open during this operation. In a similar manner telegra h si nals may be transmit ed. from. A to liould. the

repeaters tend to sing due to simultaneous transmission from A and A, the singing will cease when either A or A ceases transmitting.

Another way in which the system may be operated is to leave switches 31 and 31 closed and transmit any desired message or messages from C to D, preferably leaving switches 6 and 6"open. Only the positive halves of the waves which reach the repeater will be repeated. Should the repeater tend to sing due to simultaneous transmission from both ends of the line, the singing will stop when either C or C stops sending.

In order to repeat both .halves of the waves from C, switch 6 may now be closed to the right or to the left, and either the switch 3 or the switch 7, respectively, closed and left closed, so that the grid of the east repeater isnormally at the potential indicated at c in Fig. 2. Then both the positive halves and the negative halves of any alternatingvcurrent waves from C will be repeated by the east repeater. The system may be operated in a similar manner for transmission from east to west, it being noted that transmission occurs in only one direction at a time, and that when the key 3 or 7 at one end of the line is closed the corresponding key 3 or 7 at the other end of the line should be kept open.

A key 29 may be opened, when desired, to disconnect the local transmitting channels 20, 25 and 28 from amplifier 30.

Fig. 3 shows a repeater circuit like the circuit M of Fig. 1, but with two one-way repeaters T substituted for the one-way repeaters S of the circuit M. The upper one of the repeaters T is the east repeater, transmitting from west to east, and the lower one of the repeaters T is the west repeater, transmitting from east to west. batteries shown in the repeaters are of such voltage, and are so poled, that normally the space current is just raised to substantially the saturation ,value. In other words, in the absence of any transmission over the line to'the repeater the values of the grid potentials and of the space currents of the repeaters are in the neighborhood of those indicated by the'adjustment designated'e in Fig. 2. The path for the space current for each repeater during non-signaling periods extends from the positive pole'of the plate battery flO, to the anode, through the space current path in the repeater tube to the cathode, and through the variable resistance 41 to the negative pole of the batterylO. The output of the east repeater is connected to the line L, and to the input of the west repeater, at a point 42 adjustable between the ends of the battery of the east repeater, and at that end of the resistance 41 ofthe east repeater which connects with the The grid lLlO cathode of the repeater. The adjustment of resistance 41 and the adjustment of point 42 on battery are preferably such that normally the voltage drop in the resistance 41 is equal and opposite to the voltage of the part of the battery 40 between resistance 41 and point 42, and consequently the voltage applied to the line L and to the input of the west repeater by the output circuit of the east repeater is normally of zero value. lVhen a positive half-wave of current, or a positive potential of any kind is transmittedeastward over line L to the input of the east repeater, no change occurs in the space current of the repeater, since the current cannot exceed its saturation value. However, when a negative impulse comes over the line to the grid of the east repeater, the

space current of that repeater decreases, and consequently the voltage drop in the resistance 41 of that repeater decreases, and therefore the potential of the point 42 becomes positive with respect to the cathode of the repeater. In other words, the repeater sends a positive impulse eastward over line L and also sends a positive impulse to the grid of the WGIt repeater. This positive impulse on the grid of the west repeater does not alter the space current of that repeater, since the space current is already at its saturation value; and therefore, singing of the repeaters is avoided. The connection of the output of'the west repeater tothe line L and to the input of the east repeater is similar to the connection of the output of the east repeater to the line L and to the input of the west repeater; and the operation of the repeater circuit when the west repeater is repeating from east to west is similar to the operation of the circuit when the east repeater is repeating from west to east, as just described. If the repeater T of Fig. 3, be substituted for the repeaters-S in Fig. 1, the operation of the system of Fig. 1 will be altered to this extent: when G or C trans- Inits, with switches 6 and 6 open, only positive half waves (instead of only negative half waves, as with repeaters S) will be repeated; when it is desired to transmit both positive and negative half waves from C to the east, switch 6 and one of the keys 3 and 7, should be so manipulated as to send a steady negative potential to the grid of the east repeater T, suflicient in value to cause the repeater to operate with the adjustment indicated by the grid potential 0 in Fig. 2; similarly when it is desired to transmit both positive and negative half waves from C westward, A should maintain the grid of the west repeater T at the value indicated by c in Fig. 2; and when it is desired to transmit impulses from A eastward or from A westward, the impulses should be sent to the repeater as negative impulses.

It will be noted that whereas the repeater S, as normally adjusted, responds to an incoming positive impulse to produce an outgoing negative impulse and does not respond to a negative impulse, the repeater T as normally adjusted does not respond to a positive impulse but responds to a negative impulse to produce an outgoing positive impulse.

Fig. 4 shows a repeater circuit like that at at M in Fig. 1 except that for the two oppositely directed one-way repeaters S there have been substituted two oppositely directed one-way repeaters U, each of which, as normally adjusted, responds to an incoming positive impulse to produce an outgoing negative impulse and does not respond to a negative impulse. The grid battery shown in the repeater U is of such voltage and is so poled that the potential of the grid of the repeater is normally between the values 6 and 0 in Fig. 2, preferably at the value b.

'The output circuit of each of the repeaters U is similar to that of repeater T, but in the connection from point 42 to the line L and to the grid of the oppositely directed repeater U there is interposed a unilateral device, preferably a two electrode space current type of rectifier valve 45 with its cathode connected to the point 42. Adjustable high resistance 46 is connected between the anode of tube 45- and the cathodes of the repeaters.

To consider the operation of the two-way repeater comprising the two oppositely directed repeaters U, let it first be assumed that a positive impulse is transmitted eastward over line L to the grid of the east repeater U. The space current of that repeater will rise and therefore the potential of oint 42 will become negative. Consequenty, current will flow through resistance 46 and tube 45, and the voltage drop across resistance 46 will be applied to the line L and to the grid of the west repeater U as a negative potential. However, this negative potential applied to the west repeater will not cause singing of the repeaters, for, as will now be explained in connection with the east repeater, a repeater of the type of repeaters U will not repeat a negative impulse. Let it be assumed that a negative impulse is transmitted eastward over line L to the grid of the east repeater U. The space current of that repeater will decrease and therefore the potential of point 42 will rise; but smce current can not flow against the valve action of the rectifier 45, there will be no output impulse from the repeater. It will be noted that the operation of a repeater of the character of repeater U, when repeating a positive impulse as described above, may be confined to the straight part of the gridvoltage plate-current curve of the ampllfier, so that the impulse will be repeated with a minimum of distortion. Preferably, the impedance of. the rectifier 45 should be low in comparison to that of the remainder of the circuit connected from point 42 to the cathode of the repeater, and the internal anodecathode impedance of the repeater should approximately equal the impedance which it faces. The operation of the repeater circuit comprising repeaters U in transmitting from east to west is similar to its operation described above for repeating from west to east, and need not be further described.

Fig. 5 shows a repeater circuit like that at M in Fig. 1, except that for the two opositely directed one-way repeaters S there have been substituted two oppositely directed oneway repeaters V. Each of the repeaters V is unresponsive to positive impulses, but responds to incoming negative impulses to produce outgoing positive impulses. The repeater V is like the repeater U, except that in repeater V the rectifier in the lead connecting the point 42 to the line L is designated 45 and has its anode, instead of its cathode, connected to point 42, and except that in repeater V the voltage of the grid battery shown in the repeater is of such value that normally the grid potential of the repeater is between the values andd in Fig. 2, preferably at'the value d. The internal cathode-anode impedances of the repeater tubes and of the rectifier tubes in Fig. should preferably be such as indicated above for the corresponding space current paths in Fig. 4.

When a positive impulse is transmitted eastward over line L to the grid of the east repeater V, the space current of that repeater will rise and therefore the potential of point 42 will become negative; but since current cannot flow against the valve action of the rectifier there will be no output impulse from the repeater. If a negative impulse is transmitted eastward over line L to the grid of the east repeater V, the

' space current of that repeater will fall and therefore the potential of point 42 will become positive. Consequently, current will flow tirough resistance 46 and tube 45, and the voltage drop across resistance 46 will be applied'to the line L and to the grid of the west repeater V as a positive potential. However, this positive potential applied to the grid of the west repeater will not be transmitted through the west repeater as the west repeater, like the east -repeater, is non-responsive to positive impulses. Therefore, energy will not be transmitted from the output of one repeater V back to the input of the same repeater, and singing cannot take place. It will be noted that when the repeater U is repeating a negative impulse, itmay operate on the straight part of its grid-voltage plate-current characteristic, so that distortion will be minimized. The operation of the repeater circuit of Fig. 5 in repeating from east to west is similar to its action in repeating from west to .east as described above.

Fig, 6 shows a repeater circuit similar to t-herepeater circuit M of Fig, 1 except that for each of the one-way repeaters S of Fig. 1 there has been substituted a one-way repeating path comprising two repeaters W and S. In other words, the repeater circuit of Fig. 6 differs from that of Fig. 1 in that Fig.6 shows a repeater 'W connected between line L and the input of the east repeater S, and shows a repeater W bet-ween the output of the west repeater S and the line L. These repeaters W, are like the repeaters T of Fig. 3 except that the grid battery in each repeater W is of such voltage and polarity that the repeater \V normally operates with its grid potential at the value a in Fig. 2. The east repeater responds to a positive impulse by delivering a negative impulse to the grid of east reeater S, and responds to a negative impulse y delivering a positive impulse to east repeater S. When the positive impulse from eas t.repeater W is applied to the grid of east repeater S, the east repeater S delivers a negative impulse to line L and to the input of the west repeater S; but this negative impulse is not transmitted through the west repeater S, for reasons pointed out in the description of the operation of Fig, 1, lVhen the negative impulse from east repeater \V is applied to east repeater S, this impulse is not transmitted since repeater S does not transmit any negative impulses. It should be noted that a weak negative impulse transmitted eastward over line L to the input of the east repeater W is amplified by repeater W so that when it is applied to the grid of east repeater S as a positive impulse, it is of suflicient magnitude to quickly carry-the potential of the grid of repeater S up on the straight part of the grid-voltage platecurrent characteristic of that repeater, and thus minimize distortion.

Then a positive impulse from line L is tranmitted to the input of the west repeater S, that repeater transmits the impulse as a negative impulse, which the west repeater W transmits as a positive impulse to the line L and to the input of the east repeater V. The latter repeater transmits this positive impulse as a negative impulse, which, however, repeater S does not transmit. Therefore, singing cannot take place. lVhen line L transmits a negative impulse to the west repeater S, this repeater does not respond.

Fig. 7 is a diagrammatic showing of a two-way repeater of the general form of that of Fig. 6, but in which each of the stages Y and Z is intended to reprefent any one of several forms of repeaters. Some examplesof combinations of the repeaters S, T, U, V and W which may be used in the lit) lilo

circuit of Fig. 7, are indicated in the table of Fig. 8. It will be seen that the first combination given in the table is the circuit of Fig. 6.

Fig. 9 shows a line L for transmitting signals to and from receiving and sending apparatus at the west end of the line, this receiving and sending apparatus comprising multiplex sending means C and multiplex receiving means D of the character shown in Fig. 1, and also comprising apparatus E for sending and receiving positive and negative impulses. The apparatus E includes sending apparatus F and receiving apparatus G. Switches 76, 77 and 78 enable the apparatus C, D and E to be connected to the line as required.

A two-way repeater circuit comprising four of the repeaters S is connected in the line L for amplifying both positive and negative impulses transmitted in either direction over the line. The four repeaters are designated 50, 51, 52 and 53, and each has its grid potential adjusted to the point indicated by a in Fig. 2. Variable capacities 54 and 55 are bridged serially across the line L at the west side of the repeaters 50 to 53, and variable capacities 56 and 57 are bridged serially across the line L at the east side of repeaters 50 to 53. Repeaters 50 and 51 are cast repeaters, repeater 50 having its input leads or grid and filament leads, connected across condenser 54 and having its output leads connected across condenser 56, and repeater 51 having its input leads connected across condenser 55 and its output leads connected across condenser 56. Repeaters 52 and 53 are West repeaters, repeater 52 having its input leads connected across condenser 56 and its output leads connected across condenser 54, and repeater 53 having its input leads connected across condenser 57 and its output leads connected across condenser 55. A variable resistance 1 is connected across each of the condensers 54, 55, 56 and 57, and this resistance is of lower value than resistance 1 of Fig. 1 since resistances 1 must carry the direct current space currents of repeaters 50, 51, 52 and 53.

To consider the operation of the repeater circuit comprising the repeaters 50, 51, 52 and 53, let it be assumed that a current impulse is transmitted eastward over line L and flows downward through condensers 54 and 55. Then the voltage across condenser 54 is applied to the grid of repeater 50 as a positive impulse, and the voltage across condenser 55 is applied to the grid of repeater 51 as a negative impulse. This 'negative'impulse is not transmitted through repeater 51. The positive impulse on the grid of repeater 50 is repeated as a negative impulse, causing a current impulse to flow upwardly through condenser 56. The resulting voltage drop across condenser 56 is divided between condenser 57 and line L, and the voltage across condenser 57 is applied to the grid of repeater 53 as a negative impulse, which repeater 53 does not transmit.

Let it now be assumed that a current impulse is" transmitted eastward over line L and flows upward through condensers 55 and 54. Then the voltage across condenser 55 is applied to the grid of repeater 51 as a positive impulse, and the voltage across condenser 54 is applied to the grid of repeater 50 as a negative impulse. This negative impulse is not transmitted through repeater 50. The positive impulse on the grid of repeater 51 is repeated as a negative impulse, causing a current to flow downwardly through.condenser 57. The resulting voltage drop across condenser 57 is divided between 0011- denser 56 and line L, and the voltage across condenser 56 is applied to the grid of repeater 52 as a negative impulse, which repeater 52 does not transmit.

If positive and negative impulses are transmitted westward over line L to the repeaters 50, 51, 52 and 53, one of the west repeaters 52 and 53 will repeat the impulses of one sign, and the other one of the west repeaters will repeat the impulses of the op posite sign, in a manner similar to that in which the east repeaters function to repeat eastwardly as just described.

Variable capacities 60 and 61 are serially connected across line L where the line joins the apparatus E, and variable resistances 62 and 63 are connected across condensers 60 and 61 respectively, these resistances preferably being adjusted to equal each other. By means of a double-throw key 64 a battery 65 may be connected in circuit either with resistance 62 and a relay 66, or with resistance 63 and a relay 67. The relays 66 and 67 may be, for example, local telegraph sounders for the transmitting key 64.

An amplifier 70 has its input circuit, or its grid and filament leads, connected across resistance 62 and across condenser 60. An amplifier 71 has its input circuit connected across resistance 63 and across condenser 61. the filaments of the amplifiers 70 and 71 being connected together. A common plate battery 72 supplies space currents for amplifiers 70 and 71 through relays or sounders 73 and 74, respectively; but a common grid battery 75 normally causes the space currents to be just reduced to substantially zero value. That is, the tubes 70 and 71 have their grid potentials normally at the value 0. in Fig. 2. The potential from the negative pole of battery 7 5 is applied to the grid of amplifier 70 through resistance 62, and is applied to the grid of tube 71 through resistance 63.

When key 64 is closed on its upper contact, current flows from the positive pole of battery 65 through resistance 62 and relay 66 to the negative pole of battery 65. The resulting voltage drop across resistance 62 and condenser 60 is applied to the circuit comprising the resistance 63 and the condenser 61 in parallel with each other and in series with line L, so the grid of tube 71 is made more negative. The grid of tube 70 has been made even more negative than that of tube 71; for the voltage drop in resistance 62, which is applied to the grid of tube 70, is greater than the voltage drop in resistance 63. Since the closure of key 64 has thus caused both grids to become more negative, no space current flows in either tube, and neither of the relays 73 and 74 is operated. The impulse sent out on line L by this closure of the key 64 will flow upwardly through the condensers and 54, and will be repeated by repeater 51 as a negative impulse, causing current to flow downwardly through the condenser 57, as described above. The resulting voltageacross condenser 57 will be divided between condenser 56 and the line L, current flowing upwardly through condenser 56, eastward over the upper side of line L, downwardly through the load (not shown) at the east end of line L, and westward over the lower side of L to the lower side of condenser 57. The load at the east end of line L may be, for example, apparatus similar to the apparatus E, in which case the repeated impulse passing downwardly through condensers corresponding to the condensers 60 and 61 will aflect relays corresponding to relays 73 and 74 in a manner which will now be made apparent by describing the operation of relays 73 and 74 when an impulse passes westward over line L and downwardly through condensers 60 and 61.

The last mentioned impulse tends to make the grid of tube 70 positive and the grid of tube 71 negative. Therefore, relay 73 operates and relay 74 is not affected.

When key 64 is closed on its lower contact, current flows from the positive pole of battery 65 through resistance 63 and relay 67 to the negative pole of battery 65. The resulting voltage drop across the resistance 63 and condenser 61'is applied to the circuit comprising the resistance 62 and the coiulenser 60 in parallel with each other and in series with line L, so the grid of tube 70 is made more negative. The grid of tube 71 has been made even more negative than that of tube 70, due to the voltage drop in resistance 63, which is applied to the grid of tube 71. Thus, no space current flows in either of the tubes 70 and 71, and neither of the relays 73 or 74 is operated. The'impulse sent out on line L by this closure of key 64 on its lower contact will flow downwardly through the condensers 54 and 55, and will be repeated by repeater 50 as a positive impulse, causing current to flow upwardly through condenser 56, as described above. .The resulting voltage across condenser 56 will be divided between the condenser 57 and the line L, current flowing downwardly through condenser 57, eastward over the lower side of line L to condensers at the east end of the line which correspond to condensers 61 and 60, upwardly through these condensers (not shown) at the east end of the line, and westward over the upper side of the line, to the upper side of condenser 56. The repeated impulse passing upwardly through the condensers corresponding to condensers 61 and 60 will affect relays corresponding to relays 73 and 74 in a manner which will now be made apparent by describing the operation of relays 73 and 74 when an impulse passes westward over line L and up ward through condensers 61 and 60.

The last mentioned impulse tends to make the grid of tube 71 positive and the grid of tube 70 negative. Therefore relay 74 operates and relay 73 is not affected.

Fig. 10 is a diagrammatic showing of atwo-way repeater circuit of the general form of the two-way repeater circuit of Fig. 9, but in which each of the four blocks X is intended to represent a repeater of any one of several types of repeaters,for instance any one of the types S, T, U and V described above. When the type S is employed in the circuit of Fig. 10, that circuit becomes the same as the two-way repeater circuits shown in Fig. 9. In order to more clearly indicate the manner in which specific forms of repeaters such asS, T, U, etc., are connected in the general forms of repeater circuits shown in Figs. 7, 10, and 11 about to be described, or are arranged in any of the blocks used in the drawing to indicate repeaters, each type of repeater has its input leads designated Z and m, and its output leads designated a and o, and'these designations are also applied to the corresponding leads entering and leaving the blocks used to represent repeaters.

Fig. 11 is a diagrammatic showing of a two-way repeater circuit of the general form of that shown in Fig. 9, except that two repeaters in tandem forming a twostage repeater, are substituted for each of the repeaters S of Fig. 9. The first stage of the tandem repeaters is designated Y and the second stage Z in order to indicate that various combinations of repeaters such as the repeaters S, T, U, V, and W, for example, may be used to form the two-way repeater circuit of Fig. 11. For example, any of the combinations set forth in Fig. 8 may be used to build a specific form of circuit of the type shown in Fig. 11.

Fig. 12 shows a repeater circuit inductively connected in a line L" for repeating repeater circuit comprises four repeaters 90,

91, 92 and 93, of which 90 and 91 are the east repeaters, while repeaters 92 and 93 are the west repeaters. The grid batteries of these repeaters are of such voltages and are so poled that normally the potentials of the grids of the repeaters are at the value a in Fig. 2.

An amplifier 95 has its input connected across winding 84 through leads 96 and 97 and has its output connected to the input of a rectifier 98 through a transformer. The

rectifier 98 delivers its output to a resistance 99 through a filter 100 which functions to' smooth out the direct current E. M. F. which tube 98 delivers to resistance 99.

An amplifier 95' has its input connected across winding 87 through leads 96' and 97, and has its output connected to the input of a rectifier 98 through a transformer. The rectifier 98 delivers its output to a resistance 99' through a filter 100 which functions in a manner similar to the filter A variable portion of resistance 99 is connected in parallel to a condenser 105 in the grid circuits of repeaters 90 and 91 by means of leads 106 and 107.

A variable portion of resistance 99' is connected in parallel to a condenser ,105 in the grid circuits of repeaters 92 and 93 by means of leads 106 and 107'.

The grid circuit of repeater 90 extends from the cathode of that repeater through condenser 105 and part of resistance 99 in.

parallel, through winding 83, through the grid battery for repeater 90, to the grid of the repeater. The. grid circuit of repeater 91 extends from the cathode of the repeater through condenser 105 and a part of resistance 99 in parallel, through winding 84, through the grid battery for repeater 91, to the grid of the repeater. The grid circuit of repeater 92 extends from the cathode of the repeater, through condenser 105' and a part of resistance 99 in parallel therewith, through winding 87, through the grid battery of the repeater, to the grid of the repeater. The grid circuit of repeater 93 extends from the cathode of the repeater, through condenser 105 and resistance 99 in parallel, through winding 86, through the grid battery of the repeater, to the grid of the repeater.

The plate circuit of repeater 90 extends from the plate battery through the tube, and through winding 86. The plate circuit of repeater 91 extends from the plate battery through the tube, and through winding 87. The plate circuit of repeater 92 extends from the plate battery through the tube, lead 96, winding 84-, and lead 97, back to the battery. The plate circuit of repeater 93 extends from the plate battery through the tube, lead 96, and winding 83, back to the battery.

The grid batteries of amplifiers 95 and 95 are of such voltage and are so poled that normally the potential of the grids of these amplifiers is of the value a in Fig. 2.

The grid batteries of tubes 98 and 98 may be of such voltage and so poled that normally the potential of the grids of these tubes is of the value a in Fig. 2; or these tubes may normally pass space current.

When an alternating current is transmitted from the west over line L to the repeater circuit, the initial effect is to cause theapplication ofeither a positive or a negative potential to the grids or tubes 91 and 95 and a potential of opposite sign to the grid of tube 90. If the sign of the potential thus initially applied to the grid of tube 95 is positive, this amplifier will pass space current and cause rectifier 98 to operate, so that a highly amplified positive potential will be applied to the grids of tubes 90 and 91 through the part of resistance 99 which is included in parallel with condenser 105 in the common part of the grid circuits of tubes 90 and 91. (It may be noted that in parallel with condenser 105 is also connected a circuit composed. of con denser 105 and that part of resistance 99 across which condenser 105 is shunted.) Therefore, tubes 90 and 91 are brought to the point on' their grid-potential platecurrent characteristic corresponding to the grid potential 0 in Fig. 2. As the alternating current continues to flow from the line L to the repeater circuit,

the potential delivered from resistance 99 1 plifier, then, of course, an amplifying stage can be added between tube 95 and tube 98. The filter 100 tends to smooth out any fluctuations of the current delivered by rectifier 98 to resistance 99, and the condensers 105 and 105 tend to prevent any fluctuations in the tubes 130 to 133.

potential delivered by resistance 99 to the grids of tubes 90, 91 and 95. \Vith their grid potentials at the value a in Fig. 2, the tubes 90 and 91 now operate as an ordinary push-pull repeater, repeating currents to the cast into line L". The potentials now .applied to the grids of tubes 92 and 95 by leads 9T and'96 are negative, as are also the potentials applied to the grid of tube 93 from the output of tube 90. Therefore, tubes 92, 93, 95' and 98 do not change their space currents. I

If when alternating current first comes eastward, over line L, to the repeater circuit, it initially causes a negative potential to be applied to the grids of tubes 91 and 95 and a positive potential to be applied to the grid circuit of tube 90, this positive potential will .not be of sufficient magnitude to produce appreciable plate current in tube 90; and the next instant the potentials onthe grids of each of tubes 90, 91 and 95 will reverse in sign, whereupon tube 95 will pass space current and cause tube 98 to apply .a positive potential to the grid circuits of tubes 90 and 91 large enough to cause these tubes to operate as an ordinary push-pull amplifier, as described above. It should be noted, that, if desired, the normal potential of the grids of tubes 90 and 91 may be adjusted to a value somewhat more negative than the potential a in-Fig. 2, to insure that, in case the potential initially applied to the grid circuit of tube 90 when an alternating current" approaches the repeater from the west is of a sign tending to make that grid less negative, or positive, the grid will not actually be positive, or at least will not become sufliciently positive to interfere with the proper operation of the system.

The operation of the system in transmittingwest is similar to its operation in transmitting east.

Fig. 13 shows repeater circuits P connected in tandem in a" line L. The repeater circuit P comprises east repeating tubes 130 and 131 and west repeating tubes 132 and 133, for repeating signal impulses of both positive and negative polarities. The circuit P also com rises two tandem connected two-way ampli ers for amplifying steady direct current sent to the repeater circuit from either direction over line L for controlling the steady potential of the grids of One of these two-way amplifiers may be, for instance, of the type M of Fig. 1, or of the type shown in Fig. 4, which employs amplifiers of the type U. It is shown in Fig. 13, as of the type M, em

3 ploying two one-way amplifiers of the type S, and as connected to line L at the west side of the repeater circuit P. Therefore the control current sent from the west to the repeater circuit P over line L should be of positive sign, since the amplifier does not repeat a negative current, but repeats a 'positive potential as a negative potential. The other of the two-way amplifiers for the control currentmay then be, for instance, of the type shown in Fig. 3, which employs amplifiers T, or of the type shown in Fig. .5 which employs amplifiers V. This other control amplifier is shown as connected to line L at the east side of the repeater circuit P. This control amplifier can not repeat a positive potential, but repeats a negative potential as a positive potential. Therefore, the control current sent from the east to the repeater circuit P over line L should be of negative sign.

The repeating tubes 130 to 133 are connected in line L by means of transformers 134, 134', 135 and 135'. The transformers 134 and 134 have line windings 136 and 137, respectively, these windings being connected in parallel with each other to the line section at the west side of the repeating tubes. Transformer 134 has windings 138 and 139 inductively related to the winding 136, and transformer 134 has windings 140 and 141 inductively related to the winding 137, the windings 138 and 139 being connected in the input circuits of tubes 130 and 131, respectively, and the windings 140 and 141 being connected in the output circuits of tubes 132 and 133, respectively. The transformers 135 and 135' have line windings 142 and 143, respectively, these windings being connected in parallel with each other to the line section at the east side of the repeating tubes. Transformer 135 has windings 144 and 145 inductively related to winding 142, andttransformer 135 has windings 146 and 147 inductively related to the winding 143, the windings 144 and 145 being connected in the output circuits of the tubes 130 and 131, respecticely, and the windings 146 and 147 being connected in the input circuits of the tubes 132 and 133, respectively. Condensers 150 prevent the control current from entering the line windings of the transformers 134, 134, 135 and 135'.

The grid batteries of repeating tubes 130 to-133 are of such voltage and are so poled that normally the grid potential of these tubes is at the value a in Fig. 2, or more negative than a, but when positive direct current is sent from the west to the repeater circuit P over one line L (as, for Instance, by means of the key 3 and battery 4 shpwn in Fig. 1), the positive potential received at the repeater circuit P is applied to the grid of tube 130' over a ath including lead 155, lead .156, the grid battery, for tubes 130 and 131, winding 138, the grid of tube 130, and the grounded cathode of the tube, the lower side of line L being grounded through the lead 157 whlch connects with t e cathodes cf amp fisr M. Tips positive potential is applied to the grid of tube 131 over a path including lead 155, lead 156, the grid battery of tubes 130 and 131, winding 139, the grid of tube 131, and the cathode of tube 131 connected to ground and lead 157. This positive potential should be of such value as to raise the potential of the grids of tubes 130 and 131 to approximately the value 0 in Fig. 2. The positive potential received from the west at repeater circuit P is also applied to the amplifier circuit M, through leads 155 and 157, and is repeated by the east repeater S of the repeater circuit M as a negative potential. This negative potential is applied to the grids of tubes 132 and 133 through 'lead 160 and the windings 146 and 147,

respectively, renderingthese tubes inoperative. This negative potential from repeater circuit S is also applied to the amplifier T (or V) and is repeated by the east amplifier T (or V) to line L, through leads 161 l and 162, as a positive potential, which passes east over line L to the succeeding repeater circuit P. 1he amplification of amplifiers S (or U), and T (or V) should be such that the positive potential which they deliver to the grids of the tubes 130 and 131 (not shown) of this succeeding repeater circuit P will be such as to raise the potentials of these grids to approximately the value 0 in Fig. 2.

With the grid potentials of the tubes and 131 of the tandem connected repeater circuits P at the value 0 of Fig.2, signals,

such for instance as voice waves, transmitted east over line L from apparatus such, for example, as that shown at C in Fig. 1, will be faithfully repeated at each of the repeater circuits 1?. The operation of the system for transmission west is similar to its operation for transmitting cast; but when transmitting west the control potential should be negative, as indicated above. The west amplifier T (or V) will repeat this negative potential as a positive potential, and the amplification of thatamplifier '1 (or V) should be such that this I positive potential, passing through lead 160 and windings .146 and 147 to the grids of tubes 132 and 133, will raise the steady grids to approximately the value 0 of Fig. 2. The ositive potential from the west amplifier l[ (or V) will be repeated by the west amplifier S as a negative potential, which will pass on to line L through leads and 157, and which will also be applied to the grids of tubes 130 and 131, through lead 156 and windings 138 and 139 to render these tubes inoperative.

Condenser 163 prevents direct current from passing between lead 156 and the oathodes of tubes 130 and 131, and condenser 164 prevents direct current from passing be tween lead and the cathodes of tubes 132 and 133.

Variable resistances 165 inserted in line L at each side of the repeaters 130 to 133 are adjusted to a value such that the efi'ect of the inductive reactance of transformers 134, 134, 135, 135, upon repeater M is decreased, whereby the tendency of repeater M to sing due to reflection of the outgoing energy from repeater M back to the repeater with a reversal of potential is decreased. Resistances 165 not only reduce the efiect of the inductive reactance of transformers 134, 134., 135, 135', but also damp out transient oscillations set up in the transformer circuits.

Fig. 14 shows a line 0 having repeater circuits Q in tandem therein, with a telephone transmitting and receiving station connected to the west end of the line. It may be coilsidered that the east end of the ine is connected to a similar station. lhe telephone station comprises an artificial line AL for balancing the impedance of line 0 more or less closely, and a transmitter circuit and receiver-circuit 171 connected to the line 0 and in approximate anti-side tone relation.

The repeater circuit Q comprises east repeaters and 181 andwest repeaters 182 and 183, which are connected to line 0 by transformers and by condensers 150 in the manner in which tubes 130 to 133 are connected to the line L by transformers and by condensers 150 in Fig. 13.

A battery 185, which may be at one end of line 0, as shown, is provided for sending direct current over the line (1 to furnish grid potential, plate voltage, and filament heating current for the tandem repeaters Q.

A. variable resistance 186 is provided, for adjusting the voltage delivered to the line by the battery. A filter 187 for filtering out fluctuations of battery current is connected in the output leads of the battery,

and by means of a double-throw switch 188 circuit Q, through switch to point 196,

thence through variable resistance 197,

choke coil 198, resistances 199, 200, and 201,

choke coils 202 and 203, resistance 204, switch 205, lead 206, switch 207, resistance 208, choke coils 209 and 210, resistances 211, 212 and 213, choke coil 214, and resistance 215, to point 216, thence through lead 217 and switch 195 to the upper side of line 0, and e t thr g he pp r side of line 0 to the positive pole of the battery 185. Resistance 199 is so adjusted that the voltage drop produced therein by this dlrect current is of such value as to bring the grids of tubes 180 and 181 to substantially the value c in Fig. 2. It will be noted that the potential drop across resistance 211 is of wrong polarity to produce space current through tubes 182, 183 and consequently these tubes are inoperative as long as battery 185 is connected to repeater Q. The proper voltage for heating the filaments of tubes 180 and 181 is tapped from resistance 200; and the proper plate voltage for these tubes is tapped from resistance 201.

A lead 218 connects point 196 with the lower side of line 0 at the east side of the repeater circuit Q, through a switch 219; and a lead 220 connects the point 216 with the upper side of line 0 at the east side of the repeater circuit, through the switch 219. Thus the direct current from battery 185 is fed over line 0 east of the repeater circuit Q, to subsequent repeater circuits Q.

The direct current path at each of the tandem connected repeaters is across the line 0, in parallel with the direct current paths of the other repeaters. By throwing switch 205 to its upper contact and throwing switch 207 to its lower contact, the elements 197 to 204 can be connected in parallel with, instead of in series with, the elements 208 to 215. Filters 225 prevent alternating current from entering these elements from the line 0.

An auxiliary line 0 may parallel line 0 so that, by closing switch 188 on its lowercontacts and closing switch 195 to the left, and closing switch 219 to the right, the auxiliary line 0, instead of the line 0, may be used to transmit the direct current for energizing and controlling the repeaters.

At the telephone station (not shown) at the east end of line 0 the battery (not shown) corresponding to battery 185 should be so arranged that when it is in circuit its positive side will be connected to the lower side of line 0 (or O) and its negative pole will be connected toupper side of line 0 (or 0), whereby tubes 182 and 183 are rendered operative in a manner similar to that described above in connection with tubes 180, 181, while tubes 180 and 181 will be inoperative owing to the anodes of tubes 180 and 181 being negative with respect to their cathodes.

When the transmitter circuit 170 is sending signal energy east, the switch (not shown) at the east end of the line corresponding to switch 188 should be open, and the switch 188 should be closed. Transmission west is accomplished in a manner similar to that in which transmission east is accomplished, the switch 188 at the west end of the line being open, and the corresponding switch at the east end of the line being'closed.

It will be obvious that the general principles herein disclosed may be embodied in many organizations widely different from those specifically illustrated without departing from the spirit of the invention defined in the appended claims.

lVhat is claimed is:

1. In an electric transmission system, a pair of transmission lines, a repeater con nected between said lines comprising separate relaying means interconnecting said lines, controlling means rendering one of said relaying means responsive to electrical variations of only one sign incoming to said repeater from one of said lines, and controlling means rendering the other of said relaying means responsive to electrical variations of only one sign incoming to said repeater from the other of said lines, said control means causing each of said relaying means to so operate, in response to energy approaching said repeater, as to maintain the other relaying means nonresponsive to energy transmitted outward from said repeater.

2. An electric transmission system comprism a pair of transmission circuits, an

ampli er of the space current type having an input circuit conductively connected to one of said transmission circuits, and having an output circuit conductively connected to the other of said transmission circuits, a second amplifier of the space current type having an inputcircuit conductively connected to the output circuit of said first mentioned amplifier, and having an output circuit conductively connected in non-singing relation to the input circuit of said first mentioned amplifier.

3. In combination, two circuits, and re lay means therebetween responsive to a potential difference of one sign across one of said circuits to produce an electrical variation in said other circuit in linear relation to the potential difference across said one circuit, said means being non-responsive to a potential difference across said one circult of opposite si from said first men-- tioned potential di erence.

4. In combination, a line, and means between two sections thereof responsive to an electromotive force impulse of given sign incoming from either of said sections to produce acro:s said other section an amplified outgoing electromotive force 1mpulse of the same form but of opposite sign, said means being non-responsive to an electromotive force impulse from either of said sections of sign opposite to said given sign.

5. In combination, a line, and means between two sections thereof for transmitting signal impulses of one sign in one directio'n only between said sections and for transmit ting signal impulses of the opposite sign in only the opposite direction between said sections, said means responding. to impulses incoming from either section to produce across said other section amplified outgoing impulses of the same form and sign.

(5. In combination, a rectifying relay having a space current path therein, an output circuit for said rectifier comprising a source of electromotive force and an impedance, said source, impedance and path being connected in serial relation, and a circuit connected across said source and said impedance, said rectifier having as space current limits the value zero and'the saturation value of the space current, said rectifier being so adjusted that its space current is normally at a value in the neighborhood of but appreciably differing from one of said limiting values, and the last mentioned circuit comprising a rectifier so connected therein as to prevent flow of current therein due to change of space current in said output circuit toward said one limiting value.

7. In combination, a rectifying relay having a space current path therein, an output circuit for said rectifier comprising a source otelectromotive force and an impedance, said source, impedance and path being connected in serial relation, and a circuitconnected across said source and said impedance, said rectifier being so adjusted that its space current is normally in the neighborhood of but above zero value, and the last mentioned circuit comprising a rectifier so connected therein as to prevent flow of current therein'due to decrease of space current in said output circuit.

' 8. In combination, a rectifying relay having a space current path therein, an output circuit for said rectifier comprising a source of electromotive force and an impedance, said source, impedance and path being connected in serial relation, and a circuit con nected across said source and said impedance, said r'ectifier being so adjusted that its space current is normally in the neighborhood of but below the saturation Value, and the last mentioned circuit comprising a -rectifier so connected therein as to prevent flow of current thereindue to increase of space.

current in said output circuit.

9. In combination, a rectifying relay-haw ing an anode, a cathode and a space current path therebetween, an output circuit for said rectifier comprising a source of electromotive force and an impedance, said source, impedanceand path being connected in serial relation, and a circuit connected across said source and said impedance, said lastmentioned circuit comprising a rectifier having an anode connected tothe anode of said first mentioned rectifier.

1.0, In combination, a rectifying relay having a space current path therein, an output circuit for said rectlfier comprising a source of electromotive force and an impedance,

vent How of current therein due to change of 1 space current in said output circuit toward said one limiting value, and the impedance of said second mentioned rectifier being low in comparison to the impedance of the remainder of said Second mentioned circuit. 11. In combination, a rectifying relay having a space current path therein, an output circuit for said rectifier comprising a source of electromotive force and an impedance, said source, impedance and path being connected in serial relation, and a circuit connected across said source and said impedance, said rectifier having as space current limits the value zero and the saturation value of the space current, said rectifier being so adjusted that its'space current is normally at a value in the neighborhood of but appreciably difiering from one of said limiting values, the last mentioned circuit comprising a rectifier so connected therein as to prevent flow of current therein due to change of space current in said output circuit to- Ward said one limiting Value, the impedance of said second mentioned rectifier being low in comparison to the impedance of the re mainder of said second mentioned circuit, and the impedance of said space current path being substantially equal to the impedance which it faces.

12. A two-Way transmitting circuit, an asymmetrically transmitting path for transdnitting to said circuit, an asymmetrically transmitting path for transmitting from said circuit but normally conditioned to pre- .vent distortionless transmission of alternating current through said second path, said first ath transmitting to said second path electromotive forces of only such character as tend to maintain said second path in said condition.

13. A two-Way signal transmitting circuit, an electron tube amplifier for transmitting to said circuit, an electron tube amplifier for transmitting from said circuit, said second mentioned amplifier having its space current normally at zero value, and means so connecting said first mentioned amplifier to said circuit that the signals transmitted to said second mentioned amplifier by said first mentioned amplifier tend to maintain the space current of said second mentioned amplifier'at zero value 14. A line, means for supplying direct current to said line at times as signal impulses and at other times as steady direct current potential, means for supplying alternating current signaling energy to said line, a repeater connected in said line for repeating said direct current impulses with amplified energy, and faithfully repeating said alternating current at only the times during which said direct current is supplied to said line as a steady direct current potential.

15. The method of operating an electron discharge repeating device in a line, which comprises at times transmitting direct current signal impulses, only, over said line, at other times transmitting alternating currents, only, over said lines, and at still other times simultaneously transmitting both a steady directcurrent potential and an alternating current over said line and through said repeater.

' 16. A line, a two-Way repeater therein for repeating either positive or negative current impulses in either direction over said line, means connected to one end of said line for sending to said repeater either positive or negative current impulses, and signal indicating means connected to said end of said line and responsive tocurrentimpulses transmitted from said repeater selectively in accordance with the sign of said last mentioned impulses, but responsive to neither positive nor negative impulses sent into said line by saidfirst mentioned means.

17- A line for} transmitting alternating current and direct current, a repeater inductively connected therein, a connection for applying a steady direct current potential from said line to said repeater for condi tioning said repeater to faithfully repeat said alternating current transmitted over said line, and means shuntedaround said repeater for amplifying sald direct current direction over said line but to render said repeater incapable of repeating in the opposite direction.

19. In combination, two circuits, and means interconnecting said circuits, said means comprising a pair of electric space discharge tubes, havin input electrodes and output electrodes, ortransmitting in one direction between said circuits, a pair of electric space discharge tubes, having input electrodesand output electrodes, for transmitting in the' other direction between said circuits, and a single pair of trans formers connecting both pairs of tubes between said circuits, with the tubes of each pair in push-pull relation and with the output electrodes of each of said pairs of tubes conduct-ively connected to the input electrodes of the other pair.

20. In combination, two electric space discharge devices having anode, cathode, and control electrodes, means connecting said devices in push-pull relation, means for supplying elect-romotive force across said anode and cathode electrodes, and means for ap- I JUSTIN L. FEARING. 

